Special Counsel Robert Mueller’s investigation into Donald Trump’s Russia scandal has increasingly begun to play out in public. Three of Trump’s key campaign advisers have been arrested. One of them has already pleaded guilty. Another is providing cooperative testimony to a grand jury. Even as the probe moves closer to Trump and Mike Pence, Mueller has an ace up his sleeve – hidden in plain sight – that most observers have lost track of in the chaos.

Trump’s campaign chairman Paul Manafort, who is now under house arrest, is being as uncooperative as possible. That might change if Mueller offered him a deal that included little or no prison time. But Mueller doesn’t need to do that, because he already has Manafort’s unwitting cooperation. He’s had it for the better part of two years, because there was a FISA surveillance warrant against Manafort during and after the campaign.

There is no doubt that Mueller and his team have already taken possession of every relevant Manafort phone call that was recorded on those wiretaps. That means Mueller has lower level campaign people nailed left and right, and he’s likely already working to flip those individuals. More importantly, Donald Trump and Mike Pence are both known to have communicated with Manafort by phone, after Manafort left the campaign, in the general timeframe that the FISA warrant is known to have been active. Pence was relying on Manafort during the transition period, for reasons that cannot have been above board. Trump was calling Manafort after he took office, for reasons that likely constitute obstruction of justice or witness tampering.

Robert Mueller only needs to trap Trump and Pence into lying to him about what’s on those tapes, and that’ll be a felony right there. On the other hand, if Trump and Pence admit what they truly said to Manafort, that’ll probably take them down as well. Trump and Pence are each likely finished the minute they sit down with Mueller.